As will be better understood, the present invention is directed to transmission line phase shifters that are ideally suited for use in low-cost, steerable, phased array antennas. While ideally suited for use in low-cost, steerable, phased array antennas, and described in combination with such antennas, it is to be understood that transmission line phase shifters formed in accordance with this invention may also find use in other environments.
Antennas generally fall into two classes—omnidirectional antennas and steerable antennas. Omnidirectional antennas transmit and receive signals omnidirectionally, i.e., transmit signals to and receive signals from all directions. A single dipole antenna is an example of an omnidirectional antenna. While omnidirectional antennas are inexpensive and widely used in environments where the direction of signal transmission and/or reception is unknown or varies (due, for example, to the need to receive signals from and/or transmit signals to multiple locations), omnidirectional antennas have a significant disadvantage. Because of their omnidirectional nature, the power signal requirements of omnidirectional antennas are relatively high. Transmission power requirements are high because transmitted signals are transmitted omnidirectionally, rather than toward a specific location. Because signal reception is omnidirectional, the power requirements of the transmitting signal source must be relatively high in order for the signal to be detected.
Steerable antennas overcome the power requirement problems of omnidirectional antennas. However, in the past, steerable antennas have been expensive. More specifically, steerable antennas are “pointed” toward the source of a signal being received or the location of the receiver of a signal being transmitted. Steerable antennas generally fall into two categories, mechanically steerable antennas and electronically steerable antennas. Mechanically steerable antennas use a mechanical system to steer an antenna structure. Most antenna structures steered by mechanical systems include a parabolic reflector element and a transmit and/or receive element located at the focal point of the parabola. Electronically steerable antennas employ a plurality of antenna elements and are “steered” by controlling the phase of the signals transmitted and/or received by the antenna elements. Electronically steerable antennas are commonly referred to as phased array antennas. If the plurality of antenna elements lie along a line, the antenna is referred to as a linear phased array antenna.
While phased array antennas have become widely used in many environments, particularly high value military, aerospace, and cellular phone environments, in the past phased array antennas have had one major disadvantage. They have been costly to manufacture. The high manufacturing cost has primarily been due to the need for a large number of variable time delay elements, also known as phase shifters, in the antenna element feed paths. In the past, the time delay or phase shift created by each element has been independently controlled according to some predictable schedule. In general, independent time delay or phase shift control requires the precision control of the capacitance and/or inductance of a resonant circuit. While mechanical devices can be used to control capacitance and inductance, most contemporary time delay or phase shifting circuits employ an electronic controllable device, such as a varactor to control the time delay or phase shift produced by the circuit. While the cost of phased array antennas can be reduced by sector pointing and switching phased array antennas, the pointing capability of such antennas is relatively coarse. Sector pointing and switching phased array antennas frequently use microwave switching techniques employing pin diodes to switch between phase delays to create switching between sectors. Because sector pointing and switching phased array antennas point at sectors rather than at precise locations, like omnidirectional antennas, they require higher power signals than location pointing phased array antennas.
Because of their expense, in the past, phased array antennas have not been employed in low-cost wireless network environments. For example, phased array antennas in the past have not been used in wireless fidelity (WiFi) networks. As a result, the significant advantages of phased array antennas have not been available in low-cost wireless network environments. Consequently, a need exists for a low-cost, steerable, phased array antenna having the ability to be relatively precisely pointed. This invention is directed to providing a transmission line phase shifter ideally suited for use in low-cost, steerable, phased array antennas.